Noise pollution is unwanted or harmful sound that intrudes upon human or other faunal activity. Noise pollution is almost entirely human generated, whether by machine sources or amplified sound of human creation. Approximately ninety percent of all such intrusive sound arises from such transport devices as motor vehicles, aircraft and rail activities. Noise pollution gives rise to an assortment of adverse human health effects as well as disruption of faunal activity. Regulation of noise pollution began in a systematic way in the United States with enactment of the Noise Control Act of 1972. Within the next 22 years a number of other national governments had emulated the U.S. initiative, including Netherlands (1979), France (1985), Spain (1993), and Denmark (1994).

Noise sources

Hydraulic jackhammers and other construction equipmentare important sources of noise. Source: USAF

Surface motor vehicles, including automobiles, trucks and motorcycles, produce approximately 85 percent of all intrusive environmental noise;[1] while there is a long term trend in quieter vehicles, the expanding human population demanding more use of surface transportation outweighs the slight downward trend in individual vehicle emissions. In the year 2010 the geographic extent of noise intrusion continues to reach new highs, as motor vehicle use continues to expand into previously quiet areas of the planet. Aircraft noise, while contributing lesser total energy into the environment, poses some of the greatest human health treats, due to the extremely high sound levels produced. Other sources of noise are industrial plants; amplified music; garden and power tools and garden power; sirens and alarms.

Most of the impacts are perceived in the outdoor environment, since building structures are partially effective in shielding sound penetration to building interiors; however, important interior receptors affected by noise pollution include: factory worker exposure to industrial machinery; building interiors in the vicinity of major airports (generally within a five mile distance from flight paths); and occupants of multi-family dwellings where party walls or floor/ceiling assemblies are not effectively designed. A party wall is the wall separating two adjoining units in an apartment or condominium.

Some consider certain instances of human voices or animal sounds to be unwanted noise, but those sources are not acknowledged universally as noise pollution, as they are essentially natural sounds of the environment, unless amplified or produced in unreasonable proximity to others.

Noise propagation

Generally, sound waves propagate in all directions from a given source, although most sources have some type of directional bias. Most of the sound transmission occurs in the atmosphere, although vibration and very low frequencies can be transmitted through the Earth surface. The principal parameters influencing the intensity of sound arriving at a given receptor are: meteorological conditions, presence of intervening barriers, nature of reflective surfaces (including the ground surface) and frequency spectrum of the source.

Meteorlogical influences play an important role in sound propagation, particularly due to vertical stratification effects in temperature and wind velocity. These microclimate atmospheric layering effects induce refraction in the sound rays, such that sound intensities at the receptor may be greatly amplified or reduced, depending upon whether the sound rays are refracted upward or downward. For example a thermal profile in which warmer air is near the Earth's surface will create less dense air at the surface level and will cause sound to refract upwards, resulting in reduced sound intensity at a receptor.

Health effects

Diagram of the inner ear, where much of the hearing loss due to noise pollution takes place. Source: Gray's Anatomy, 1918

Noise health effects are consequences of elevated sound levels, and may manifest as physiological as well as psychological impairments. Some of the chief health effects include hypertension, ischemic heart disease. Some studies have suggested changes in the immune system, and limited data attributing birth defects to noise exposure have come forward.[2] Although some presbycusis may occur naturally with age, in many developed nations the cumulative impact of noise is sufficient to impair the hearing of a large fraction of the population over the course of a lifetime.[3] Also, noise exposure has been known to induce tinnitus, hypertension, vasoconstriction and other cardiovascular impacts. Beyond these effects, elevated noise levels can create stress, increase workplace accident rates[4] The most significant causes of exposure to sound levels causing adverse health effects are vehicle and aircraft noise, industrial noise and prolonged exposure to amplified music.

Although some presbycusis (hearing loss) may occur naturally with age,[5] in many developed nations the cumulative impact of noise is sufficient to impair the hearing of a large fraction of the population over the course of a lifetime. There are two basic causative types of health effect impacts: (a) those arising from very short term, but high intensity sound, and (b) those arising from long term (measured in decades) exposure to moderate levels of sound. The first type might manifest in extremely close encounter with jet engine backblast, and could lead to temporary or permanent hearing impairment. The long term exposure is more clearly associated with hypertension as well as annoyance impacts.

Annoyance and behavioral effects

Noise pollution causes a large array of annoyance effects and behavioral change in humans and animals These range from decreased learning rates, inducement of hostility, frustration threshold reduction to speech interference and sleep disturbance. Arguably,ny of these impacts may rise to the level of health or medical stature.

Noise pollution often causes speech interference, most notably when intrusive levels exceed 63 dBA, the typical level of soft speech. In fact, there is a remarkable phenomenon that occurs in humans[6] as well as numerous animal species,[7] whereby vocalization is unconsciously altered in intensity but also pitch and cadence, in order to achieve more intelligible speech in the presence of interfering noise.This reflex was first noted by the French physician Étienne Lombard in 1909, and is known as the Lombard Reflex. The phenomenon is involuntary, as proven by the fact that it is a reliable technique for detecting people feigning a hearing impairment.

Mitigation and regulation

U.S. Capitol, site of enactment of the landmark legislation, 1972 Noise Control Act. Source: Kevin McCoy

Environmental and workplace noise may be mitigated with a variety of physical and operational measures; correspondingly there is a variety of federal and local government regulations that enforce limits of noise exposure.

Mitigation measures fall into the categories of environmental noise and industrial noise in the workplace. With regard to environmental noise, the chief sources being transportation, there are several common techniques to reduce human exposure to excessive sound levels. For aircraft noise, there have been programs in place for decades to reduce the sound level emissions of jet engines; however, there are also programs for flight operations management to minimize nighttime flights and to utilize take-off and approach paths which limit the exposure of sensitive land uses in the vicinity of airports.[8] In the case of roadway noise, the basic techniques involve reduction of engine, tire and aerodynamic noise, but also rely upon operational controls of speed reduction and avoidance of high grades that require intensive engine use in residential areas. Braking and acceleration noise can be reduced by efficient design of traffic signal synchronization. Also, there is a variety of geometric techniques for the optimum design of urban freeways using well chosen areas for cut, fill and sound barrier design. As a last resort, the use of well insulated buildings can further protect building occupants by insuring the choice of high performance glazing and complete design of the exterior building skin.

The U.S. Federal Government was the first to establish specific standards for noise/land use compatibility within the Noise Control Act of 1972. Other countries have since followed this model, and many local governments have added stricter codes for building design and for land use designations. In the State of California, for example, each city is required by law to produce a Noise Element of the General Plan that sets forth specific methods of insuring that city wide sound levels are compatible with land uses throughout each municipality.[9]